1. Field of the Invention
The present invention relates to semiconductor light-emitting devices and, more particularly, to a semiconductor laser device for oscillating a laser beam of a visible light range.
2. Description of the Related Art
Recently, of semiconductor laser devices, a great deal of attention has been paid to a laser for emitting a laser beam of a visible light range, e.g., a red light beam having a wavelength of 670 to 680 nanometers. For example, a gain waveguide type semiconductor laser for emitting a red light beam has a gallium arsenide (to be referred to as "GaAs" hereinafter)-based III-V Group semiconductor laminated structure. Typically, this laminated structure has a double-heterostructure, which consists of a first cladding layer constituted by an N type indium/gallium/aluminum/phosphorus (to be referred to as "InGaAlP" hereinafter) layer formed above a GaAs substrate of an N type conductivity, an active layer formed on the first cladding layer, and a second cladding layer constituted by a P type InGaAlP layer formed on the active layer.
Current-blocking layers are formed on the second cladding layer, and are constituted by N type GaAs layers. These current-blocking layers define an elongated, i.e., stripe-like waveguide opening. A P type GaAs contact layer is formed so as to cover the current-blocking layers. The contact layer is formed thick enough to make its top surface substantially flat. Conductive layers serving as laser electrodes are formed on the two opposite surfaces of such a laser laminated structure. When a device having such a structure is excited, a current is blocked by that portion of a PN-inverted layer between the current-blocking layers and the second cladding layer which corresponds to the opening, thereby providing laser beam oscillation along the length of the opening (i.e., oscillation of a stripe-shaped laser beam).
In the above-described laser structure, a P type semiconductive layer is formed between the second cladding layer and the current-blocking layers. The P type semiconductive layer has a band gap corresponding to an intermediate value between the band gap of the second cladding layer and the contact layers. This layer functions to reduce a change in band gap between the second cladding layer and the contact layers, and is generally known as an "intermediate band-gap layer". The current-blocking layers define the above waveguide opening on the surface of the intermediate band-gap layer. In order to form such a waveguide opening, an N type GaAs layer which is formed on the intermediate band-gap layer in advance is etched by a known etching technique.
The conventional laser having the above-described laminated structure, however, suffers from the following problem. When laser beam oscillation is to be performed, incomplete current blocking in the intermediate band-gap layer causes an excitation current to undesirably spread in a lateral direction in the intermediate band-gap layer. Such spreading of a current in the intermediate band-gap layer decreases the oscillation efficiency of a laser beam.
In order to prevent the "current spreading" phenomenon, the carrier densities and thicknesses of the second cladding layer and the intermediate band-gap layer must be controlled with high accuracy. If the carrier density of the intermediate band-gap layer is decreased to prevent "current spreading", the series resistance of the device is undesirably increased. It is difficult to control the thickness of the intermediate band-gap layer with high accuracy for the following reasons. In the process of etching the N type GaAs layer to form the waveguide opening, it is very difficult to control the etching process so as to accurately stop etching progress at the top surface of the underlying layer (i.e., the intermediate band-gap layer) in consideration of the current technical level of etching. If overetching occurs in the intermediate band-gap layer due to insufficient etching precision, an initially set proper thickness of the intermediate band-gap layer cannot inevitably be obtained. Since an overetching amount is not necessarily constant in a manufacturing process, even if the thickness of the intermediate band-gap layer is set a little larger than necessary so as to compensate an overetching amount, the intermediate band-gap layer of a manufactured device does not necessarily have an optimal thickness. If the initial thickness of the intermediate band-gap layer is changed for each device, manufacturing yield is decreased. If the material for the intermediate band-gap layer is replaced with a gallium/aluminum/arsenic (GaAlAs) material, the problem of "overetching" can be solved. However, the carrier density of the intermediate band-gap layer is decreased. As a result, the series resistance of a semiconductor laser device is increased, thus posing another problem.